Since there is no virtual rj11 port this would be tough. I would standup a physical fax workstation/server/appliance and have it integrate with SBS/Exchange via the network. This is the only way I have seen that customers were able to work in a virtual environment

A) Attach an harware fax/modem to virtual serial port
B) Use a gateway
C) Subscribe an online account to send fax across internet

A)
I've tested ESX with virtual serial port connected to dedicated virtual machine and attacched US Robotics Modem connected to rj11.
I've used old fax software from symantec to send fax.
The only issue was that sometimes faxes are not sent or received well.

B)
You can use hardware gateways from lan to rj11 or isdn or T1 line.
For example ZETAFAX in the virtualzed environment propose some hardware gateway, but you can choose any of that type of devices.

C)
If you don't want to be mad about the ability of sending and receiving fax you can use any of the outsorced services existing on the internet. For example we use www.messagenet.it to send and receive faxes. For sending you simply send an e-mail to specific address with attachment and the service send the fax for you.
You can also ask your ISP or phone provider if they can provide you this type of service and at what price.